This invention is in the general field of multi-ply tissue products as well as methods and apparatus for bonding tissue plies to make such products.
Various tissue products are made by laminating multiple layers of tissue. These layers or plies must be bonded to prevent them from delaminating or floating apart when they are converted into the final product or when they are used by the consumer.
Ply bonding of the individual tissue sheets into multi-ply layers is usually achieved by embossing them on converting equipment without applying adhesive. In this process, two or more tissue webs are simultaneously unwound and fed through a nip formed between male and female embossing rolls to emboss or crimp the webs and thereby bond them together. Often in making such products as napkins, the webs are embossed only around the perimeter of areas that will be cut into the individual napkins.
In a different process, tissue plies may be adhered using a chemical adhesive rather than by embossing. For example, Bjxc3x6rk U.S. Pat. No. 5,466,318 discloses a process for laminating webs using a water-based adhesive.
Controlling ply bonding is important and difficult. Inadequate, excessive or inconsistent ply bonding can jam complex, high-speed machinery, generate high waste, and provide unacceptable product. The strength of bonding by embossing (without adhesive) may vary depending, among other things, on water content or dryness of tissue webs, and on ambient air humidity. While bonding by embossing sometimes can be improved by increasing the pressure on the embossing rolls, such pressure can wear out the embossing rolls more quickly, particularly the female roll, which is usually a softer roll made of composite material onto which a pattern is impressed by the opposing, male, engraved metal roll. In addition, the journals and bearings of both embossing rolls can also wear out prematurely if subjected to increased pressure over a prolonged period.
The use of adhesives can avoid some of the problems of embossing, but adhesives also cause problems, such as xe2x80x9cthrough bondingxe2x80x9d or xe2x80x9cblockingxe2x80x9d, in which adjacent laminates bond to one another, and, prevent unwinding of the laminate product from its roll. Non-uniformity of bonding also causes problems, such as wrinkling of the tissue and bad printing.
As noted, the bonding process may be part of the overall process for making the final multi-ply product. For example, multi-ply tissue products, particularly napkins, may be printed, e.g., by a letterpress or a flexographic process. In a typical letterpress printing process, a multi-ply web is first embossed, and then the central unembossed area of individual napkins is printed. In an alternative flexographic printing process, unbonded webs may be first run through a flexographic printing tower and then crimped with an embossing nip.
While the invention has broader application, we refer specifically to certain markets, particularly high quality napkins such as the party goods, food service, and other specialty napkin markets. These markets may require cloth-like character and feel, high wet strength, and the ability to be decorated with high quality printing. Increasingly, these markets have expressed preference for quality and complexity provided by flexographic printing.
It can be difficult to achieve cloth-like character and feel while also achieving satisfactory bonding of two or more plies of tissue to make one sheet of napkin tissue that will not come apart either when used by the consumer or when subjected to flexographic printing. The napkin must not de-ply, it must stay in one piece like a cloth napkin would. However, the use of adhesives and other additives to develop the resistance to ply separation tends to make the product stiff and destroy the cloth-like feel.
Another product requirement in this field is the need of strength while wet, which can be critically important to high quality paper napkins. Non-wet strength napkins may pull apart when subjected to moisture, such as spilled liquid or a wet glass, rendering then unsatisfactory for the high quality markets. Chemicals normally employed to impart wet strength (e.g., urea, phenol-formaldehyde) can reduce ply bond-strength, which is already problematic, and can make the napkin stiffer.
With regard to decorated or printed napkins, certain napkin machines (e.g., those made by Servotec and Hoberna), which use flexographic printing have stringent ply requirements. As noted, these flexographic machines print the napkin first and emboss it afterwards, in contrast with letterpress napkin machines, which first emboss the edges of the napkin so that the plies of tissue are crimped together, after which the napkin is printed. In the flexographic process, it is critical that the plies be held together without embossing while the napkin is being printed. Floating plies as the tissue enters the printing stage result in jams and high waste together with machine downtime. In addition, the application of the ink causes the top layer of the tissue to expand if the plies are not well bonded, resulting in wrinkling, badly printed napkins, and extremely high costs from waste.
These problems are sometimes addressed by the use of much heavier napkin tissue (as has been done in Europe where some of the flexographic machines were introduced). Heavier tissue is more expensive.
We have discovered spray bonding processes and apparatus that provide substantially improved efficiency and process control of tissue bonding. The resulting adhesively bonded intermediate product is improved, particularly for use in flexographic printing applications. The final product may be a paper napkin, a paper towel, a medical disposable towel, a table cover or other similar tissue product. The cloth-like feel achieved by the invention is suitable for a wide range of markets, including not only markets where softness is important but also markets where a certain amount of product stiffness is required, such as the markets in which napkins are forced into plastic wraps containing cutlery (napkins made and packaged by xe2x80x9cAustin-Gordonxe2x80x9d or xe2x80x9cGordonxe2x80x9d-type machines)
In particular, the invention provides a well-bonded napkin product that can be used in converting apparatus, particularly flexographic printing apparatus. This ply-bonded tissue is constructed of lighter weight material that is less costly than the previously used tissue (particularly in Europe), yet the tissue exhibits printability and a cloth-like feel. Cloth-like character and feel, high wet strength, printability and low cost are achieved in a reliably ply-bonded product without the need to emboss before printing. The result is markedly enhanced ply bonding of the tissue layers and efficient and high quality printing, without detracting from the cloth-like character and feel that the use of adhesives can cause.
One aspect of the invention generally features a multi-ply adhesively bonded tissue roll with an extremely limited amount of waste at the core, which results from the absence of bonding or poorly controlled bonding during start-up of the bonding process. The roll comprises a bonded region that extends to within at least xc2xe inch of the core. (This xc2xe inch measurement is made from the outer diameter of the core, which is typically about 4 inches to the point in the product at which adhesive bonding begins.) Adhesive contact between the plies is substantially continuous over the bonded region of the roll (i.e., at radiuses greater than xc2xexe2x80x3), and the strength throughout the bonded region of the roll is high, substantially without through bonding in that region. While bond strengths of at least 100 mg/cm are achieved, in one particular embodiment, the bond strength is at least 400 mg/cm.
In another aspect of the invention described below, we have found that careful process control permits lower adhesive loading and the use of a pressurized nip to force the webs together without unacceptable adhesive build-up on the nip rolls, particularly for two-ply tissue product. Strong two-ply adhesion can be achieved with lower adhesive loading, thus avoiding problems caused by inconsistent adhesive loading while at the same time enabling better control over winding tension (because the nip controls tension transmitted upstream). Thus, this aspect of the invention features methods of producing an adhesively bonded two-ply tissue product by: spraying an adhesive mixture upwardly onto one of the moving webs from at least one nozzle array positioned in a spray location below one of the webs; compressing the webs together as they run through a nip formed by two nip rolls positioned at a compression point downstream of the spray location; and winding the bonded two-ply tissue product onto a roll under tension using a winder which pulls the two-ply product, exerting tension on it. In this aspect of the invention, it is practical to use a pressurized nip to force the adhesively bonded webs together. The distance between the spray location and the nip is selected to permit sprayed adhesive to partially but not completely set during travel over that distance at operating web speeds. The webs may be forced together with nip rollers that have enough pressure to substantially confine winder tension to the nip-to-winder portion of the web path, as opposed to transmitting winder tension upstream to the parent reels (i.e. the reels on which the tissue stock has been supplied to the bonder), thereby providing improved control over winder tension. Because the spray location is carefully controlled, it is possible to use such a nip without unacceptable adhesive build-up on the nip rolls.
In the above process, the adhesive is considered to be partially but not completely set when it has what is known in the field as xe2x80x9cgreen tackxe2x80x9d. At that stage, the adhesive is damp to the touch, but it does not transfer to another surface, so the adhesive will contact bond, but it will not migrate so far through the tissue web that it causes through bonding as the bonded webs are wound into a roll.
Typically, each tissue web is unwound over its own carrier roll, and spray nozzle(s) are positioned between the carrier rolls, particularly adjacent to and downstream of the rear carrier roll. For example, the web travel distance between the nozzles and the nip is more than 140 (and even more than 200 but less than 500) inches.
In a third aspect, the invention features methods of producing tissue product with more than 2 (usually 3 or 4) plies. The method uses at least two spray nozzle arrays, a rear array to spray the bottom of an upper web and a forward array to spray the bottom of an intermediate web. The multiple webs are tensioned by a winder around a compression roll (usually but not necessarily a nip roll as described above) to compress the webs. The web travel distance between the two nozzle arrays and the nip roll is controlled so that adhesive is partially but not completely set when it reaches the roll. Both nozzle arrays are positioned as discussed above. Typically the forward nozzle array will be more than 140 inches (preferably more than 200 inches) from the compression roll.
Preferably, webs are run at a speed of well over 1500 feet/min (more me preferably 2,300 feet/min or higher), and the dwell time for adhesive applied to the web is between 0.3 and 1.3 seconds. Dwell time is the period from application of adhesive to the web until the adhesive treated web reaches the nip.
Alternatively, to make three- or more ply product, the number of sprayer locations may be two less than the number of plies being bound. For example, four moving webs are positioned vertically to form an upper web, a upper-middle web, a lower-middle web, and a bottom web respectively, and only two arrays of adhesive spray nozzles are used: a first array positioned in the rear location to spray upwardly onto the bottom surface of the upper-middle web; and a second array positioned in a third location to spray upwardly onto the bottom of the lower-middle web. Similarly, the method may be used to make three-ply tissue by positioning three moving webs as an upper web, a middle web, and a lower web, respectively, and using only one adhesive nozzle array, which is positioned in the rear location to spray upwardly onto the middle web.
A significant advantage of the method is that it can be used in series with high-speed tissue-forming equipment, i.e., reels of tissue webs are provided by forming them on equipment including a dryer, and winding up the resulting webs, which then are bonded at a rate which exceeds the rate at which the webs are produced. When we say that the webs are bonded at a rate which exceeds the rate at which the webs are produced, we take into account down time at the beginning of a run for loading reels of freshly made unbonded tissue and time at the end of a run to unload bonded multi-ply tissue. We also take into account similar down time for the tissue production process. Finally, we take into account that at least two and sometimes three or four tissue webs must be made to produce a single multi-ply bonded web so that, as a rough approximation and if all other things were equal, a two-ply bonding process could run at xc2xd the speed of the tissue manufacturing process without becoming a bottleneck. In general, the web speed exceeds 1500 feet/min (preferably over 2000 feet/min) during a significant portion of the bonding process.
Tissue making relies on management of the creping process, in which dried tissue is scraped from a heated drying cylinder to which it has been adhered. Creping is improved generally when the drying cylinder is hot and the tissue is relatively dry at the point of creping. The above described bonding process is advantageous in that it can bond relatively dry webs, without adding or retaining moisture at the tissue-forming stage to accommodate the needs of the bonding process.
Control over the spray process may include controlling the adhesive mixture flow with respect to web travel speed, for example, to maintain a substantially constant (+or xe2x88x9220%) overall rate of addition of dry adhesive per unit area of web at different speeds (preferably the addition rate is between 5 and 25 (most preferably between 5 and 20) pounds/million square feet of multi-ply web). The method may also include controlling the air pressure used to spray the mixture at different web travel speeds.
Typically, the method includes at least two phases, a running phase and a substantially slower transition phase (e.g., starting up or slowing down the winder). The amount of liquid adhesive delivered per minute is substantially proportional (xc2x120%) to the web speed during both the running phase and the transitional phase. The atomizing airflow pressure or flow rate may also be controlled responsive to changes in web travel rate between the running and the transition phase.
Multiple nozzles may be positioned on supports in a shower housing which includes a drain for over-spray. The shower housing includes a damper that is movable from a first damper orientation that prevents nozzle spray from reaching the moving web to a second damper orientation in which spray reaches moving web. During the start-up phase, the damper is positioned in the first orientation so that web is not bonded. The shower housing may be raised from a an inactive position during idle periods when the web is slowed or stopped to a second active position. Adhesive may be purged from the nozzles in a cycle that includes introducing flush water and/or airflow into the shower nozzle.
Preferred adhesives are aqueous mixtures, e.g., of carboxymethyl cellulose; polyvinyl alcohol; or starch. The spraying process control described above enables the use of higher (for example at least 13%) solids in the liquid adhesive, which in turn results in shorter drying times.
After the adhesive is applied, the webs may be forced together by a pair of rolls (e.g. calender rolls) that are in contact as the two webs pass through. For example, these rolls may be biased together to form a pressure nip. The bonded tissue plies are then wound up on a winder downstream of the nip, and tension on the winder is substantially maintained downstream of the nip and is not substantially communicated upstream of the nip.
The above method is practiced using apparatus that includes:
A. a rear reel stand and a rear carrier roll above the rear stand, forming the beginning of an upper web path;
B. a forward reel stand and a forward carrier roll above the forward stand, forming the beginning of a lower web path, the upper web path converging with the lower web path at the forward carrier roll;
C. an adhesive applicator system for applying adhesive to the bottom of tissue moving in the upper web path, the adhesive applicator system comprising:
i. an array of spray nozzles positioned at a spray location below and generally transverse to the upper tissue path, and oriented to spray generally upward, toward the bottom surface of the upper tissue, the first location being between the rear and the forward carrier rolls;
ii. an air pressure source connected to provide air to the spray nozzles
iii. an adhesive source connected to provide a flow of adhesive to the spray nozzles, and
D. a pair of nip rolls positioned downstream from the spray head array and the forward carrier roll, the nip rolls being in both the upper and lower tissue paths, the nip rolls forming a nip for compressing the upper and the lower tissue together
E. a winder downstream of the nip for winding up bonded tissue product, the winder being designed to have a running speed within a predetermined range,
the nip being spaced apart from the spray location at least 230 inches, to permit sprayed adhesive to partially but not completely set during travel over that distance at speeds in the predetermined range.
The location of the array of spray heads is controlled as described above. For example, the nip is spaced apart from the first (rear) location by at least 200 inches to permit sprayed adhesive to partially but not completely set during travel over that distance. Specifically, the array is located between the rear and the forward carrier rolls.
The apparatus may also include: A. an adhesive pump providing a flow of adhesive to the spray heads; B. an air pump providing air pressure to the spray heads; C. a sensor for sensing the speed of travel of at least one of the tissue webs, which provides a web speed signal to a signal processor; and D. an adhesive flow control system for controlling the flow of adhesive to the spray heads; the adhesive flow control system includes a flow rate monitor, a signal processor which receives a signal from the monitor and provides signals to an adhesive flow motor, responsive to the web speed signal, so that the adhesive delivery rate is varied in response to web travel rate.
When more than two tissue webs are being bound, the apparatus includes at least one intermediate reel stand and intermediate carrier roll, forming the beginning of an intermediate web path, the intermediate carrier roll being positioned between the rear and the forward carrier rolls. The apparatus may also include additional arrays of spray heads positioned at a second (forward) location below and generally transverse to the intermediate web path, and oriented to spray generally upward, toward the bottom surface of the intermediate tissue.
The apparatus can operate fast enough to process the output of the web-forming equipment substantially without backlog or slowing of that equipment.
A signal processor includes circuitry to control the amount of adhesive flow supplied to the nozzles substantially proportional to the web travel speed, so as to provide a substantially constant overall ratio of dry adhesive per unit area of web at different web speeds. The controller also includes circuitry to establish the other method controls described above, such as the controls on adhesive flow, airflow damper position, and shower housing position. The controller also controls a purge cycle in which flush water and/or airflow is directed into the nozzle.
As noted, the above process and apparatus use tissue that is very dry without need for further modification. The process avoids the need to emboss as a bonding process. The process permits effective use of high speed tissue forming machinery. The process improves utilization and reduces waste, particularly waste from discarding the beginning and end of rolls due to improper adhesive loading at transition speeds.